1. Field of the Invention
The present invention relates to a wafer holder and a method of producing a semiconductor wafer with ground sides and a rounded edge, in which the wafer holder is used.
2. The Prior Art
The production of a semiconductor wafer comprises the cutting of the semiconductor wafer from a crystal and a series of material-removing machining steps. Suitable as cutting tools are Inner Diameter, band and wire saws. If a wire saw is used, a plurality of semiconductor wafers can be cut simultaneously from a crystal. Wire-sawed semiconductor wafers are severely contaminated after the cutting operation because of the sawing suspension used in wire sawing. Regardless of the cutting method used, a semiconductor wafer cut from a crystal has damage in the near-surface region which has to be removed by subsequent material-removing machining steps. These machining steps are also necessary in order to provide the semiconductor wafer with a rounded edge, with as smooth a surface as possible and with parallel sides. After the cutting operation, the thickness variation of the semiconductor wafer, the flatness of its side surfaces and the structure of its edge still do not meet the requirements necessary for the subsequent processing of the semiconductor wafer to produce electronic components. The material-removing machining steps normally include the edge rounding, lapping, etching and polishing of the cut semiconductor wafer.
It has also been proposed to replace the lapping in which both sides of the semiconductor wafer are machined simultaneously by two grinding steps. These two grinding steps are to be carried out one after the other wherein one side of the semiconductor is ground in each case. U.S. Pat. No. 5,400,548 describes such a grinding method. During the grinding, the semiconductor wafer lies on the support of a wafer holder or chuck. Standard wafer holders have a support made of porous ceramic material. The semiconductor wafer is fixed to the wafer holder by vacuum suction. That side of the semiconductor wafer is ground which is opposite to the side lying on the wafer holder. During edge rounding, the edge of the semiconductor wafer is ground.
The grinding of the sides of the semiconductor wafer achieves a good parallelism for the surfaces of the sides. On the other hand, it has been determined that the flatness of the sides thereby obtainable does not meet the necessary requirements. In order to achieve the desired flatness, subsequent material-removing machining steps, such as etching and polishing, have to take a more expensive form. Accordingly, the material removal to be included in the calculation for the production of a semiconductor wafer with desired wafer shape increases so that the wafer yield for a certain crystal length decreases.
During grinding or edge rounding of a semiconductor wafer, undesirable material splintering must be expected on that side of the semiconductor wafer situated opposite the wafer holder. This splintering has been referred to as crow's feet. Another phenomenon which impairs the quality of the semiconductor wafer relates to local depressions or dimples on the sides of the semiconductor wafer which can be produced during the grinding of the sides. Crow's feet and depressions are likely to occur, if the semiconductor wafer to be machined is contaminated with particles. Wire-sawed semiconductor wafers, in particular, therefore have to be laboriously cleaned prior to grinding or edge rounding.